Many discussions on the pros and cons of evolutionary theory focus on the question whether a complex structure – like the eye – can evolve through natural selection or has been created by an intelligent designer. The answer is, of course, beforehand indisputable as it depends on what one wants to believe. There it is, no hard prove exists in matters like religious conviction and long-term natural selection.
So it all depends on argument and cogency.
It already started in the 18th century when the British theologian William Paley (1743-1805) introduced one of the most famous metaphors in the philosophy of science, the image of the watchmaker: “…when we come to inspect the watch, we perceive. . . that its several parts are framed and put together for a purpose, e.g. that they are so formed and adjusted as to produce motion, and that motion so regulated as to point out the hour of the day; that if the different parts had been differently shaped from what they are, or placed after any other manner or in any other order than that in which they are placed, either no motion at all would have been carried on in the machine, or none which would have answered the use that is now served by it. . . . the inference we think is inevitable, that the watch must have had a maker -- that there must have existed, at some time and at some place or other, an artificer or artificers who formed it for the purpose which we find it actually to answer, who comprehended its construction and designed its use.” (Paley, 1802). Living organisms, Paley argued, are even more complicated than watches, "in a degree which exceeds all computation". How else to account for the often amazing adaptations of animals and plants? Only an intelligent Designer could have created them, just as only an intelligent watchmaker can make a watch: “The marks of design are too strong to be got over. Design must have had a designer. That designer must have been a person. That person is GOD”. (Paley, 1802)

Seventy years later the famous book on natural selection by Charles Darwin was published.  Advocates of intelligent design eagerly point at Darwin’s apparent demonstration of doubt when he wrote: “To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.” (Darwin, 1872). This sentence is often cited by creationists to show that even Darwin did not really believe that the eye could evolve through natural selection. However, they neglect to cite the rest of the paragraph: “Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound.” (Darwin, 1872).

Modern evolutionists never hesitate to follow Darwin’s suggestion on how step by step the eye could have evolved through natural selection. The steps usually coincide with the presentation of a now-living animal with “eyes” like described by such a step. Unfortunately, the presentation often has two weak points: the omission to mention the functional advantage of a new step and the disregard of compound eyes.
First there must be light sensitive cells on the outside of a body. This is not hard to imagine as many organisms developed light sensitive molecules (e.g. chlorophyll in cyan bacteria and plants). It only gives perception of the presence of light (on or off) and is nowadays found in some Annelids like the earthworm (Lumbricus terrestris).
A second step is the concentration of a number of light sensitive cells in a bowl-shaped depression or on a hillock. Both structures have the advantage that the individual light sensitive cells have different orientations which enables to register the direction of the incoming light. The bowl-shaped depressions are found in present free-living Platyhelminthes (flatworms).
From the hillock-structure the many compound eyes may have evolved which are much more abundant in the animal kingdom than our more familiar vertebrate eye. Compared to the bowl-shaped eye, the compound eye must become very large to obtain similar resolving power (Lythgoe, 1979). On the other hand, compound eyes have evolved into highly specialized sense organs that enable to discriminate up to twelve primary colours and several types of polarized light (the Mantis shrimp Gonadactylus smithii; Chiou et al., 2008; Kleinlogel & White, 2008).
A further depression of the bowl-shaped structure together with an increase of the number of light sensitive cells and a diminution of the opening may have evolved into a sort of  camera obscura with the advantage of more detailed visual perception at the cost of, however, brightness. Such an eye can be found in present squid like Nautilus.
Trimming and covering of a gradual thickening cuticula over the light sensitive cells in both the camera eye and the compound eye improved detail discrimination with retention or even enlargement of the incoming light. Such eyes are now known among many representatives of vertebrates, molluscs and arthropods.
Of course, a contemporary development of the ciliated and rhabdomeric photoreceptors must also have taken place to enable the more sophisticated ways of modern vision: the origin of various kinds of photopigments to enable colour vision, the evolution of polarisation sensitivity and the development of rods (scotopic vision) and cones (photopic vision). Besides, the more complex retinal layers have beyond question evolved after the original development of mere light sensitive spots in order to enhance contrast improve movement detection.
Nevertheless, all this is mere speculation. It cannot be proven to be true. It only seems plausible. Even among scientists there is dispute. Most biologists believe(d) in a repeated evolution of various eye-types. Only fairly recently molecular evidence revealed a single evolution of all present eyes descending from one proto-eye (Arendt, 2003).

A couple of years ago the design historian Jan Michl held a lecture on design and evolution. Advocates of intelligent design now have to face a new opponent since Michl claims that if Paley’s famous metaphor (see above) roves anything, it is the existence of evolution. It is quite unthinkable that Paley’s watch has no developmental history of its own. In Michl’s words: “Paley endeavors to play down the fact that the watchmaker’s profession, and his art was built on foundations laid down by others: that his watchmaker learned his craft through apprenticeship, that the watchmaker, in producing the watch was dependent on the division of labor, i.e. on contributions from other makers, that both the watch and the watchmaker profession have a long history of their own, that previous watches were different from the ones of Paley’s time, that still earlier there were no mechanical watches, only clocks, and that earlier still there were no mechanical clocks at all. In omitting to suggest that both the watch and the watchmaker have their own history, i.e. in presenting only a static, synchronic view of both, Paley, just as his ID followers some 200 years later, lets the complexity, rationality and functionality of the watch look as if it was a sovereign, autonomous creation that popped out from the watchmaker”s lone head.” (Michl, 2006). Actually, Paley’s argument for a Designer decides in Darwin’s favour: it totally agrees with the theory of natural selection. It is a bit embarrassing that even a prominent evolutionist like Richard Dawkins has not been aware of this: “Richard Dawkins, the well-known, brilliant and hard-hitting British champion of Darwinism, with a predilection for baiting religious people, called in 1987 one of his renowned books The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design, a title obviously referring to - and obviously mocking – Paley’s watchmaker argument. However, Dawkins chooses to ridicule only the second part of Paley’s argument, the idea of God as a solitary designer behind complex organic adaptations in nature, while being apparently perfectly happy with the first part of the argument, the idea of a solitary human designer behind a complex artifact. In a committed evolutionist this is certainly puzzling.” (Michl, 2006).
As mentioned before, the discussion probably is far from over. Rest me to state: will be continued.

www.geocities.com/Athens/2360/eng.intelligent.html
http://www.talkorigins.org/indexcc/CB/CB301.html
http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html
http://www.blackwellpublishing.com/ridley/a-z/Evolution_of_the_eye.asp
http://images.google.nl/imgres?imgurl=http://bp1.blogger.com/_nvWgMkm
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http://www.eyedesignbook.com/ch2/eyech2-abc.html

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